The genes in question control a type of white blood cell known as natural killer cells, or NK cells. Humans aren't the only primate with NK cells. Chimpanzees have them as well. But Peter Parham, a professor of cell biology at Stanford University, realized that there must be some key differences between the ways NK cells in chimps and humans behave. "There are a number of fairly major infections that infect humans but don't infect chimpanzees," he says. "HIV is one, malaria seems to be another." Not only do natural killer cells play an important role in preventing disease, they also play a role in controlling blood flow between a mother and her developing fetus. As a pregnancy progresses, blood flow becomes more critical. But there seems to be a trade-off. The kind of NK cells that are good for getting lots of blood to the developing fetus are not as good for dealing with infection, and vice versa. Parham looked closely at the kinds of NK cells most common in humans, and compared them with the NK cells most common among chimpanzees. The chimpanzee system seems to be much more optimized for dealing with infection. The human system, on the other hand, seems to be optimized for getting lots of blood to the developing fetus so our big brains can grow the way they're supposed to. That may have something to do with why we are smarter than chimpanzees. I wonder if differences in NK cells may also explain the variation in human intelligence? In general, tropical populations tend to be less intelligent than non-tropical ones but also seem to be better protected against diseases such as malaria.
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